1. Field of the Invention
This invention generally relates to a lithographic plate having a tin-plated steel support and a process for preparing such a plate. In particular, the invention relates to the depassivation and electrolytic application of copper or chromium to a tin-plated steel support so that, after exposure and development, a lithographic plate having oleophilic zones of a photopolymer or copper and hydrophilic zones of chromium or tin may be created.
2. Description of the Prior Art
Lithographic printing plates now available on the market generally comprise either a stainless steel support or an aluminum support. For long-run plates, it is generally preferred to use stainless steel supports because of their high mechanical resistance and their ability to withstand a high number of impressions, on the order of several hundred thousands. However, a stainless steel support is an expensive support. Therefore, it often is preferred to use aluminum supports which are less expensive than stainless steel supports and which, in addition, have the advantage of being hydrophilic. However, these supports have a lower mechanical resistance as compared to steel supports and have a tendency to crack, which limits the number of impressions of a lithographic plate employing aluminum supports.
Use of lithographic plates of tin-plate is known from British Pat. No. 1,232,395. To make lithographic plates, the tin-plated support is coated with a photosensitive layer which, after selective exposure through an original, is stripped to make the tin selectively bare, which is etched until the underlying steel appears. The plate is then immersed in a copper-coating bath to copper coat the bare steel zones.
Finally, the photosensitive layer selectively protecting the tin is removed by a suitable developer. Thus, there is obtained a lithographic plate having a support of steel coated in the hydrophilic zones with a layer of tin and in the inkphilic (oleophilic) zones with a layer of copper.
This embodiment of a lithographic plate has several drawbacks:
Such a plate can only be copper-coated after having been exposed through the original to be reproduced, which inconveniently requires the presence of a copper-coating bath at the location of the printer performing the exposure and development. Further, immersion of the tin-plated support in the etching bath requires a protection for the tin layer on the back of the plate so that it will disappear and cause oxidation of the steel during use of the plate. Finally, cost factors require that the tin on the back of the plate must also be protected during the copper-coating operation to prevent depositing of copper on the back of the plate because of the high cost of this metal.
Another use of tin-plated steel as a lithographic plate support has been described in Belgian Pat. No. 762,992. The tin-plated steel is used as a polymetal lithographic plate because it makes it possible, according to the description of this patent, to obtain a sufficiently fine surface state. In this way, layers of copper and, then, of chromium deposited on the plate are free of surface defects. Thus, a steel support with a more or less controlled roughness can be used, the surface tin layer, optionally remelted, making it possible to obtain a support surface having a satisfactory condition for lithographic printing.
The problem encountered with the above-cited art is that it fails to disclose a tin-plated steel support and method of preparation for such a support which is simple, inexpensive and results in a hydrophilic lithographic plate.